The invention relates to a continuous casting method, especially a continuous steel casting method, and to an apparatus for the practice of the method.
For the economical practice of the continuous casting process, very high rates of descent are necessary. The resulting long solidification stretches are a peculiar characteristic of the continuous steel casting process. The steel strand solidifies commonly from the outside towards the center. On this account, and due to the leading of oriented crystals, which is known as bridging, solidification voids are formed in the core. The rest of the melt, in which the segregating elements are concentrated, also solidifies in the core. Thus, core segregations develop, which become visible in the form of a black spot on etched transverse sections.
When the continuous casting process is used for the manufacture of high-grade steels, it becomes necessary to reduce core porosity and core segregation. For this purpose, it is the general practice in the casting of high-grade steels to operate at low temperature, at a slow pouring rate, with appropriate spray cooling, and with careful alignment of the continuous casting apparatus. Some of these measures, however, result in a negative influence on the output of the continuous casting plant. Consequently there has been a search for better solutions.
One possibility is the use of an electromagnetic rotating field to act upon the still molten inner part of the strand. For this purpose, induction coils are disposed above, below or also around the continuous casting mold, which have the purpose of keeping the molten metal in movement, so as to reduce the segregation, promote degassing, prevent the incorporation of slag and bring about a change in the shape of the pool of the molten metal in the mold. In addition to rotating fields by which the molten core of the strand is set in rotation, linear fields are also used for the purpose of stirring the molten metal along horizontal or vertical axes. Core segregation and core porosity can be reduced in this manner. Inclusions, which in the case of curved-strand continuous casting installations collect mainly on the inside of the curve, are uniformly distributed over the cross section.
If induction coils are disposed underneath the mold, it becomes difficult to incorporate the coils into the strand guiding framework. Spray cooling usually has to be omitted in the vicinity of the coils. If break-offs occur, the coils are easily destroyed.
If induction coils are to be provided around or inside of the continuous casting mold, design difficulties are again involved. Furthermore, additional measures are necessary in order to make it possible for the magnetic field to penetrate through the wall of the copper mold.
In addition to electromagnetic stirring means for the production of a stirring or turning movement in the molten core of the strand, rotatory strand casting methods have become known in which the mold is set in rotation with the strand. In this manner a rotatory movement of the molten core is likewise produced. These methods, however, can be applied only in vertical continuous casting systems producing strands of round cross sectional shape.
German Auslegeschrift No. 2,163,928 discloses a metal strand casting method, especially a continuous casting method, in which the melt is set in rotation about the strand axis within the continuous casting mold, either by means of electromagnetic fields or by the rotation of the mold and of the strand, and an inert liquid gas is fed onto the bath surface in the continuous casting mold. Liquid nitrogen or liquid argon can be used as the liquid gas. By combining the feeding of an inert gas in liquid form with the rotation of the melt by electromagnetic fields or by rotating the mold and the strand, an improvement in quality is achieved. The gas must be delivered in liquid form onto the bath surface, because only then can an excellent distribution of the liquid gas be achieved when it contacts the bath surface, and this is essential to achieving the desired result. The liquid gas must be delivered at a rate which will not disturb the state of the surface of the bath.
In this method, too, either an induction coil or a rotation of the mold and steel strand are necessary for the production of the rotatory movement of the melt, so that this method has the same disadvantages as described above.